1. Field of the Invention
This invention relates generally to displays, and in particular to adjusting the duration of pulses for pulse width modulated displays.
2. Background
Pulse width modulated displays, in various forms, have been important products of the display industry for over 30 years. Originally the constant luminance displays were the primary type in use, although today both plasma display panels (PDP) and digital light processing (DLP) are the most common types found in wide distribution. U.S. Pat. No. 3,590,156, to Easton, provides an excellent early example of these types of displays.
Pulse width modulated displays are prone to a number of visual artifacts. The exact nature of each artifact depends heavily on the implementation of the pulse width modulation and the nature of the display in which it is implemented. These can be roughly divided into static image artifacts (gray scale errors) and motion artifacts (perception errors originating in the pulse width modulation). Pulse width modulated liquid crystal displays may also show a lateral field artifact which is present whether the image is moving or static and which has origins similar to those for moving artifacts.
One additional source of error in a pulse width modulated display is that it is often difficult to modulate the display with exactly the correct pulse width to achieve the desire gray level exactly. This limitation arises because of the limitations in the hardware implementation of the pulse width modulation concept.
A pulse width modulation technique in which the pulse width modulation is determined by the length of time a row is left in a given memory state is described in co-pending U.S. Patent Application, Publication No. US 2003/0210257, entitled “Modulation Scheme for Driving Digital Display Systems” incorporated by reference herein in its entirety. In the technique described, after a row of the display is written, the row is not visited again until a display data scheduling device writes the row again to establish a new data state for a different segment of the pulse width modulation. Each row can be addressed in turn as it is not necessary to change more than one row at a time.
FIG. 1A is a diagram illustrating an example of pulse width modulation. In FIG. 1A, a 1952×1112 element display is shown. The numeric coordinates represent position on the display with (0, 0) representing the upper left hand corner of the display. The coordinate system is arbitrary. The arrow 102 represent the direction of movement over time of the dividing line 104a-d between bit planes. The values of individual pixels are not presented here.
FIG. 1B is a diagram illustrating another example of pulse width modulation. In FIG. 1B, the lower left portion of a display is illustrated. The arrow 112 represent the direction of movement over time of the dividing line 114a-e between least significant bit planes.
The display as implemented sequences through each bit plane in an order determined by many factors, but principally by the need to control artifacts. The sequence is not explicitly important to this discussion. One important point is that each bit plane is written during a single sequence and then the sequence starts again on a different row.
Conceptually, the modulation method is relatively efficient of bandwidth. The use of regular row writes and the use of spacing to determine gray scale values facilitates the reduction, or elimination, of peaks and valleys in the required bandwidth without lowering system performance to an unacceptable level. In practice the surface area of a display presenting a particular higher order bit plane is roughly proportional to its weighting relative to the area displaying the least significant bit (LSB). The ratios are disturbed by the electro-optic curve of the liquid crystal cell but the principal is clear.
The modulation technique itself has a temporal coarseness or temporal resolution limitation in that the time for each bit plane can only be adjusted in one row increments. This limitation can be dealt with by such techniques as spatial and temporal dither, but these techniques add complexity to the calculation of the data to be displayed.
Therefore, there is a need for improved systems, apparatus, and techniques for providing a more deterministic method for modulating a display.